Did some research on temp. sensors

[Southpa]

I've learned that coolant temperature sensors and their respective gauges are considered a matched set. The sensor also called a "variable resistance thermistor" allows more voltage to pass to the gauge as the temperature rises. Said another way, the resistance across the unit decreases with temperature. The gauge is a voltmeter that monitors the voltage released by the sensor. And it is calibrated to the specific voltage output curve of the sensor.

My problem is that my stock gauge is reading too high and I needed to find out where the fault was. Prior to that I was using an aftermarket sensor/gauge package. The sensor and gauge came as a unit and the gauge was calibrated to the sensor's output. I found that my normal operating temperature ranged between 190 and 200 degrees F with this setup using a 195 thermostat.

OK, it was a cheap setup and finally broke down (sensor corroded) on me so I decided to install the original stock sensor and hook it up to my stock gauge. BTW, I originally switched over to the aftermarket package because I wanted to see some real numbers and wasn't sure if I had overheating problems or what. I replaced the rad and hoses after that and things cooled down nicely.

Onwards... I decided to do a little testing with a couple of stock GM sensors I had laying around. I got a pot of water boiling and put them in with a thermometer. I then measured the resistance across their respective casings and electrodes at increasing temperatures and compared those readings with the chart found in this website:

http://www.diy-efi.org/gmecm/compone...o/sensors.html

Results were that both units were pretty close to the chart readings. When installed in the truck BOTH sensors read in the HOT zone on my stock gauge. And I am positive I have no overheating problems in the coolant system.

I then tested one of those sensors with the aftermarket gauge and found that the gauge did not budge at all. This was a result of incompatibility between the heavy-duty GM sensor and the cheap gauge. The sensor retained too much resistance even at 195 degrees to allow voltage to the gauge. The aftermarket sensor/gauge worked on a much smaller resistance/voltage scale than the stock setup.

What did I learn from all this?? My gauge is not working right. I have heard that when they go they read too high. There was mention that this problem could be corrected by putting a "shunt" at the back of the gauge. Anyone heard of this before? I'd like to keep the stock setup but, if I have to, I'll spend some money and buy a quality package.

[mikep]

OK first off I am going to say you have gone through a lot of trouble to avoid spending $5 at the junkyard for a temperature gauge.

Next I am going to say that the stock temp sender is calibrated to provide 68-82 ohms of resistance at ( thats 75 ohm nominal with a 7 ohm spread ) at 220 degrees F . In other words you should check the temp sender you are using for a reference against these values because not all senders have the same resistence value even if they are GM senders. Different years have different values and anything after 72 wont give you the right reading. Heat it up to the boiling temp of a mild antifereeze solution and take an Ohm reading of the resistance to ground . It should be about 75 ohm plus or minus a few.

It doesnt really matter what you are using for a sender as long as it corresponds to what your actual coolant temp is. For example I am using a 70 ohm at 220 nominal sender made for 63-70 Volvos because my coolant temp is pretty steady at 180 as I have the radiator from hell , a full shroud and a 180 thermostat so I stay at 180 no matter the load or ambiant temperature. I went with the Volvo thermocouple because it fit the hole in the side of the World Product heads I am running and it was the resistence value that put my temp needle straight up at 180.